FCC main fractionator

Naphtha wash section in a fluid catalytic cracker (FCC) main fractionator. 

Specialised units are used to simulate complex fractionation processes in petroleum refining. Typical configuration consists of a main column with pump-around and side strippers (Fig. 3.14). Among applications, we may cite pre-flash tower, crude atmospheric distillation, or Fluid Catalytic Cracking (FCC) main fractionator. 

A wet environment exists in the FCC gas plant. Water comes from the condensation of process steam in the main fractionator overhead condensers. In the presence of H S, NH3, and HCN, this environment is conducive to corrosion attacks. The corrosion attack can be any or all of the following types [2] ... 

The FCC reactor pressure is usually controlled at the suction of the wet gas compressor. The reactor pressure is the wet gas compressor suction pressure plus pressure drop through the main fractionator system. 

The HO FCC unit effluent must first be processed in an FCC style main fractionator. The main fractionator must remove catalyst fines from the heavy-oil product. The main fractionator also produces a light cycle oil and an overhead gas that is primarily light hydrocarbons and gasoline. The overhead of the main fractionator can be further processed via a wet-gas compressor. The gas is then stripped with the gasoline absorbed via a lean-oil absorber, followed by amine treatment and finally a caustic wash. The combined effluents are sent to compression and into a series of contaminant removal beds and hydrogenation steps. 

Spiral heat exchangers have been used with success in refinery fluid catalytic cracking (FCC) unit service to cool the main fractionator bottoms product, where they have proved to be less subject to fouling than other types of exchangers. Although one client commented that they had needed to make awkward repair welds between internal channels. 

We classify kinetic models according to the chemical entities that makeup the model. Typically, the entities or lumps are boiling point lumps or yield lumps, grouped chemical lumps and full chemical lumps. Early kinetic models consist entirely of yield lumps, which represent the products that refiner collects from the main fractionator following the FCC unit Figure 4.4 shows a typical kinetic model based on yield lumps by Takatsuka et al. [9]. Many similar models have appeared in the literature. The models differentiate themselves based on their number of lumps. Models may contain as few as two [10] or three lumps [11] and as many as fifty lumps [12]. We note that models with more lumps do not necessarily have more predictive capabilities than models with fewer lumps [6]. 

Figure 4.37 and Figure 4.38 show the process flow diagrams (PFD) for the FCC unit and downstream fractionation units that we will use to buUd the model in question. We extensively discussed the features and operating issues associated with this type unit in Chapter 2. In the context of this chapter, we also build models for the main fractionator and associated gas plant... 

Figure 4.38 Main Fractionator associated with the FCC unit.

The FCC configuration requires choosing the riser configuration, number and type of regenerators and catalyst configuration. We may also specify the additional downstream fractionation in the form of a simplified main fractionator for the FCC... 

The main objective in FCC catalyst design is to prepare cracking catalyst compositions which are active and selective for the conversion of gas-oil into high octane gasoline fraction. From the point of view of the zeolitic component, most of the present advances in octane enhancement have been achieved by introducing low unit cell size ultrastable zeolites (1) and by inclusion of about 1-2 of ZSM-5 zeolite in the final catalyst formulation (2). With these formulations, it is possible to increase the Research Octane Number (RON) of the gasoline, while only a minor increase in the Motor Octane Number (MON) has been obtained. Other materials such as mixed oxides and PILCS (3,4) have been studied as possible components, but there are selectivity limitations which must be overcome. 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

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